Method of continuously producing heat treated electrolytic tinplate



April 14, 1964 s. M. NEWBRANDER ETAL 3,129,150 METHOD OF CONTINUOUSLY PRODUCING HEAT TREATED ELECTROLYTIC TINPLATE Filed Feb. 27, 1958 EFFECT OF HEAT TREATING LOIb/BB ELECTROLYTIC TIN PLATE RATE OF UNTREATED z TIN PLATE SAMPLES 7 AVERAGE CORROSION INVENTORS STA/VLf Y M; IVEWBRA/VDER EDW/IV J. SMITH JOHN R. SMITH ATTORNEYS TEMPERATURE OF HEATING "C portions of the same coil varies substantially.

United States Patent Office 3,129,150 Patented Apr. 14, 1964 3,129,150 METHOD F CONTENUUUSLY PRODUCING HEAT TREATED ELECTROLYTIC TINPLATE Stanley M. N ewhrander and Edwin J. Smith, Steubenville, (thin, and John R. Smith, Weirton, W. Va., assignors to National Steel Corporation, a corporation of Delaware Filed Feb. 27, 1958, Ser. No. 718,048 1 Ciaim. (Cl. 204-37) The present invention relates to the treatment of tinplate, and more particularly, to a method of treating electrolytic tinplate to increase the shelf life and improve the uniformity of shelf life. The invention further relates to an improved method of producing electrolytic tinplate and to an improved electrolytic tinplate.

The processes most commonly used for the manufacture of tinplate are the hot dip process and the electro lytic process. By either process, ferrous metal base may be given a tin coating of desired thickness, but generally electrolytic tinplate is produced with a thinner coating of tin than is hot dip tinplate. Whatever the thickness of the tin coatings deposited on ferrous metal base by the hot dip and electrolytic processes, in each instance it is desirable that the tinplate exhibit good corrosion resistance and uniformity of corrosion resistance so as to result in a tinplate characterized by a long average shelf life and uniformity of shelf life.

Both long average shelf life and uniformity of shelf life are characteristics particularly desirable in a high quality tinplate for use in fabricating containers. For example, long average shelf life is necessary where the container fabricated from the tinplate is to be used in the preservation of perishable products, storage of liquids, etc., over extended periods of time. If the corrosion rate is high enough to result in a short average shelf life, filled containers fabricated from the tinplate will corrode after a relatively short period of time to result in swelling due to the formation of hydrogen in the container, or where the corrosion is highly localized, to result in swelling and eventual loss of the contents due to perforation of the container. The uniformity of shelf life is also of importance. It the shelf life of the tinplate varies appreciably in samples taken from different runs, or in samples taken from different portions of the same run, then the indicated average shelf life gives very little assurance of the actual shelf life of any one filled container fabricated therefrom. Thus, while most of the filled containers may be stored safely for periods of time up to the average shelf life, a number of the containers will fail and often at relatively short periods of storage as indicated by the average shelf life, thereby resulting in loss of the contents.

The average shelf life of electrolytic tinplate samples taken from different runs made under apparently identical operating conditions often varies radically. In many instances, the shelf life of samples taken from different The art has long recognized these disadvantages of electrolytic tinplate. However, prior to the present invention asuitable process for improving the average shelf life of electrolytic tinplate and at the same time the uniformity of shelf life has not been available to the art.

It is an object of the present invention to provide an improved method of producing electrolytic tinplate.

It is a further object of the present invention to provide a method of improving the average shelf life and/or the uniformity of shelf life of electrolytic tinplate.

It is still a further object of the present invention to provide a method of improving the average shelf life and/or the uniformity of shelf life of electrolytic tinplate by heat-treating the tinplate at an elevated temperature below the refiow temperature of the tin coating.

It is still a further object of the present invention to provide an improved electrolytic tinplate which is characterized "by increased average shelf life and/or uniformity of shelf life.

Still other objects of the present invention and the advantages thereof will be apparent to those skilled in the art by reference to the following detailed description and the drawing, which graphically illustrates the effect on the corrosion rate of heat-treating electrolytic tinplate.

In accordance with the present invention, the average shelf life of electrolytic tinplate is increased and/or the uniformity of shelf life is improved by heat-treating the tinplate at an elevated temperature below the reflow temperature of the tin coating. The period of heattreat: ment at a selected temperature of treatment may be of importance for optimum results in some instances, as will be explained more fully hereinafter.

To obtain the benefits of the present invention, the electrolytic tinplate must be heat-treated at an elevated temperature subsantially above room temperature but less than the reflow temperature of the tin coating, i.e., less than about 231.8 C. At a temperature of heattreatment at or above the refiow temperature of the tin coating, the oiiginal shelf life is decreased or the degree of improvement is not such as to result in a generally superior tinplate. A very satisfactory temperature range which yields good results within relatively short periods of heat-treatment has been found to be a temperature between about C. and less than the reflow temperature of the tin coating. Best results within short periods of heat-treatment, such as 1 to 30 minutes, are usually obtained by heat-treatment at a temperature of about 225 C.

The period. of the heat-treatment at the selected temperature should be of sufficient duration to result in a substantial decrease in the original corrosion rate, and thus a substantial increase in the original shelf life. Generally, the minimum period of heat-treatment produc ing a substantial increase in shelf life at the selected temperature of heat-treatment varies inversely with the temperature of heat-treatment. For example, periods of heat-treatment as short as 5 minutes, 1 minute and 30 seconds usually will result in some increase in shelf life at temperatures of heat-treatment of 175 0., 200 C. and 225 C., respectively. A minimum period of heattreatment in excess of 5 minutes, such as 15 to 30 minutes or longer, should be used at elevated temperatures substantially above room temperature but lower than about 175 C. The upper limit on the period of heat-treatment is not usually of importance within practical periods of time, but generally periods of heat-treatment in excess of about 30 minutes are not necessary or preferred. When heat-treating at a temperature between about 175 C. and less than the refiow temperature of the tin coating, the period of heat-treatment may be between 1 and 30 minutes for satisfactory results. For best results, in instances where the selected temperature of heat-treatment is approaching 175 C., then the period of heattreatment should be approaching 30 minutes; while in instances where the selected temperature of heat-treatment is approaching the reflow temperature of the tin coating, then the period of heat-treatment should be approaching 1 minute. However, as pointed out above, periods of heat-treatment somewhat longer than 30 minutes do not appear to adversely affect the product to any appreciable extent and may be used in many instances.

The drawing graphically illustrates the effect of heattreatment at varying temperatures and for varying periods of time on five 1.0 lb./BB electrolytic tinplate samples having, as indicated by line F, an average corrosion rate of about 1.87 mg. Fe/cmP/ 24 hrs. before heat-treatment, which indicates an original average shelf life of about 121 days. It may be seen from curves B, C, D and E that the corrosion rate decreases markedly after heat-treatment in accordance with the present invention and, since the average shelf life increases with a decrease in corrosion rate, it is apparent that the shelf life of the tinplate also increases greatly after heat-treatment. In addition, the uniformity of the shelf life is greatly improved. When the period of heat-treatment is appreciably less than 1 minute, e.g., 30 seconds as shown by curve A, then the decrease in corrosion rate and the increase in shelf life is not sufiicient to result in a superior tinplate, although a substantial improvement is evident. It is apparent from curves B, C, D and E that superior results are obtained at temperatures of heat-treatment of about 175 0., 200 C. and 225 C. when the period of heat-treatment is at least about minutes, 3 minutes and 1 minute, respectively. It is also evident from curves B, C, D and E that when the temperature of heat-treatment exceeds the reflow temperature of the tin coating, a generally superior tinplate is not produced.

The particular manner of effecting heat-treatment of tinplate in accordance with the present invention is not of importance and any suitable method may be used. For example, one very satisfactory method of heat-treating electrolytic tinplate in accordance with the present invention has been found to be an oven which is maintained at the desired temperature. Preferably this is done in such a manner as to insure uniform heating of each sheet of tinplate over its entire surface area. The sheets of electrolytic tinplate are allowed to remain in the oven for the desired period of heat-treatment and are then removed after the expiration of the heating period and allowed to cool. If desired, the heat-treated tinplate then may be packaged for shipment without subsequent treatment. It is apparent that an oven or the like may be used in a continuous method of heat-treatment, such as by continuously passing the electrolytic tinplate through a zone heated to the proper temperature and at such a rate as to provide the proper residence time.

While an oven or the like is the prefrred method of heat-treating electrolytic tinplate in accordance with the present invention, still other methods may be employed, such as a suitable oil bath maintained at a temperature within the desired range. The electrolytic tinplate to be treated may be immersed in such an oil bath in such a manner as to assure uniform heating. The tinplate is allowed to remain in the heated oil bath for the desired period of heat-treatment and is then removed and cooled. When desired, the oil adhering to the treated tinplate may be removed in any suitable conventional manner, such as by a scrubbing and washing operation.

The method of the present invention is most beneficial in treating flow brightened electrolytic tinplate which is considered to be of normal to poor quality. For example, heat-treatment of flow brightened electrolytic tinplate which is considered to be above average or of good quality in the untreated state will not result in the degree of improvement as will treatment of flow brightened electrolytic tinplate falling within the normal to poor classification. However, the process of the present invention is also of great value in improving the uniformity of shelf life of electrolytic tinplate and it may be desirable in many instances to heat-treat electrolytic tinplate in accordance with the present invention regardless of the apparent average shelf life since the manufacturer will be assured of a high quality uniform product.

When the method of the present invention is incorporated in a prior art continuous electroplating line for producing flow brightened electrolytic tinplate, the heattreatment step of the present invention is preferably carried out immediately after the flow brightening and quenching steps. Thus, the flow brightened electrolytic tinplate prior to the heat treating step and subsequent to the heat treating step has a bright reflective flow brightened tin surface substantially free of a permanent adherent hard organic coating, and the heat treatment is the first heat treatment under the conditions of temperature and time defined herein to which the flow brightened electrolytic tinplate is subjected following flow brightening. However, electrolytic tinplate may be treated in accordance with the present invention at any convenient time. For example, electrolytic tinplate may be heattreated before or after fabricating the tinplate into containers and the like.

The manner in which heat-treatment of electrolytic tinplate results in increased shelf life and uniformity of shelf life is not fully understood at the present time. Whatever the reason for the improvement, it apparently is not related to an increase in the amount of alloy layer present. For example, heat-treatment of electrolytic tinplate in accordance with the present invention results in only a very small increase in the amount of alloy layer present, while tinplate treated at temperatures above the reflow temperature of the tin coating results in a greatly increased amount of alloy layer and the tinplate does not exhibit the degree of improvement noted when the treatment is in accordance with the present invention.

The foregoing detailed discussion and the following specific examples are for the purpose of illustration only, and are not intended as limiting to the spirit or scope of the appended claim.

EXAMPLE I Specimens of electrolytic tinplate were prepared from a coil of 1.0 lb./BB flow brightened electrolytic tinplate. This particular coil of tinplate had an average prune pack shelf life of 109 days.

The following procedure was followed for heat-treating and testing the above specimens:

(1) The specimens were cut to size (3 x 12 cm., with the 12 cm. dimension being across the line of plating) and identified by stenciling.

(2) Each specimen was then heat-treated in an oil bath maintained at the desired temperature for varying time intervals.

(3) After receiving the desired heat-treatment, the specimens were immediately removed from the oil bath and degreased.

(4) The heat-treated specimens were tested to determine the corrosion rate of each sample and the results calculated in mg. Fe/cmF/24 hrs. Predicted shelf life for each sample based on the corrosion rate may be obtained from the following:

Corrosion rate (mg. Fe/emfi/M hrs.)

The results of heat-treating the above specimens at varying temperatures and over varying periods of time are tabulated below in Table 1.

6 Table 3 ALLOY GROWTH WITH TEMPERATURE AND TIME OF HEATIN Table 1 Time lb./BB Sn in Tempera- Corrosion Temperature Heated, C. Heated, Alloy Layer Specimen N0. ture Time Rate (mg. Minutes Heated, Heated Fe/cmfi/ 0. 24 hrs.)

0 .036 0. 0 .036 1. 30 175 1 .033 175 1min 2.43 10 175- .033 175 2. 33 200 1 .036 175 5 min 1. 54 200 5 .038 175 min 0.49 225. 1 .036 1.89 225 5 .048 200 30 sec 2. 40 200. 1 .047 200 1 min 1. 59 230 5 .060 200 3min 0.32 250 1 .069 200 5 min 0. 30 250. 5 .116 200 10 min fi 8 23 It may be observed from the above tabulated data, taken 225 13: 1 with the data of Example I, that the decrease in corro- 225 5min gg sion rate upon heat-treatment of electrolytic tinplate in 230 30 Sec 58 accordance with the present invention is not related to an 230 1111111 0.27 incre S 230 3mm n 0.24 a e n the amount of alloy layer 230 5min Q30 What 1s claimed is: 250 15 Sec -gg A method of continuously producing electrolytic tin- 250 plate consisting essentially of the steps of continuously 328 8-22 electroplating ferrous base metal strip with a coating of 250 tin, continuously heating the resulting tinplate strip above EXAMPLE II The procedure of Example I was repeated with the exception of substituting a solid prune pack testing procedure for the corrosion rate test of Example I. The electrolytic tinplate used in preparing the samples for this example was identical with that of Example I.

The results are tabulated below in Table 2.

Table 2 Shelf life (days Temperastorage at 378 C.) Specimen No. ure Time heated Heated 0.) Before After heating heating 218-232 100 226 218-232 152 214 218-232 131 171 218-232 147 186 218-232 158 248 218-232 123 208 162-177 111 90 162-177 120 122 162-177 144 120 Refiow 124 116 Reflow 140 130 EXAMPLE III Tinplate samples were prepared from 1.0 1b./BB electrolytic tinplate as in Example I, and then heat-treated at temperatures varying between 175 F. and 250 F. for 1 or 5 minutes. The amount of alloy layer present on each sample was then determined. The results are recorded below in Table 3.

the refiow temperature of the tin coating, continuously cooling the tinplate strip below the refiow temperature of the tin coating to produce flow brightened electrolytic tinplate, and continuously heat treating the flow brightened electrolytic tinplate at a temperature from about C. to about 230 C. over a period suificient to increase the shelf life, the minimum period of the heat treatment of the flow brightened electrolytic tinplate varying between about 30 seconds at 230 C. and about 5 minutes at 175 C., the flow brightened electrolytic tinplate prior to the heat treatment and subsequent to the heat treatment having a bright reflective flow brightened tin surface which is substantially free of a permanent adherent hard organic coating, and the heat treatment being the first heat treatment under the recited conditions of temperature and time to which the flow brightened electrolytic tinplate is subjected following flow brightening.

References Cited in the file of this patent UNITED STATES PATENTS 1,423,686 Schulte July 25, 1922 1,435,260 Russ Nov. 14, 1922 2,274,963 Hopper Mar. 3, 1942 2,369,748 Nachtman Feb. 20, 1945 2,381,778 Schoonmaker et a1 Aug. 7, 1945 2,418,088 Nachtman Mar. 25, 1947 2,503,217 Prust Apr. 4, 1950 2,661,328 Smith Dec. 1, 1953 2,770,872 Kadell Nov. 20, 1956 2,820,747 Fredrickson Jan. 21, 1958 FOREIGN PATENTS 729,914 Great Britain May 11, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0. 3 129 150 April 14 1964 Stanley M, Newbrander et a1.

It is hereby certified. that error appears in the above numbered patent ,reql iring cqrreetiqn and that the said Letters Patent should read as corrected below Column 6 Table 3 first column, line 9 thereof for "200" read 230 Signed and sealed this 22nd day of September 1964.,

SEAL) lmestz ERNES w. SWIDER' EDWARD J BRENNER \ltesting Officer Commissioner of Patents 

1. A METHOD OF CONTINUOUSLY PRODUCING ELECTROLYTIC TINPLATE CONSISTING ESSENTIALLY OF THE STEPS OF CONTINUOUSLY ELECTROPLATING FERROUS BASE METAL STRIP WITH A COATING OF TIN, CONTINUOUSLY HEATING THE RESULTING TINPLATE STRIP ABOVE THE REFLOW TEMPERATURE OF THE TIN COATING CONTINUOUSLY COOLING THE TINPLATE STRIP BELOW THE REFLOW TEMPERATURE OF THE TIN COATING TO PRODUCE FLOW BRIGHTENED ELECTROLYTIC TINPLATE, AND CONTINUOUSLY HEAT TREATING THE FLOW BRIGHTENED ELECTROLYTIC TINPLATE AT A TEMPERATURE FROM ABOUT 175*C. TO ABOUT 230*C. OVER A PERIOD SUFFICIENT TO INCREASE THE SHELF LIFE, THE MINIMUM PERIOD OF THE HEAT TREATMENT OF THE FLOW BRIGHTENED ELECTROLYTIC TINPLATE VARYING BETWEEN ABOUT 30 SECONDS AT 230*C. AND ABOUT 5 MINUTES AT 175*C., THE FLOW BRIGHTENED ELECTROLYTIC TINPLATE PRIOR TO THE HEAT TREATMENT AND SUBSEQUENT TO THE HEAT TREATMENT HAVING A BRIGHT REFLECTIVE FLOW BRIGHTENED TIN SURFACE WHICH IS SUBSTANTIALLY FREE OF A PERMANENT ADHERENT HARD ORGANIC COATING, AND THE HEAT TREATMENT BEING THE FIRST HEAT TREATMENT UNDER THE RECITED CONDITIONS OF TEMPERATURE AND TIME TO WHICH THE FLOW BRIGHTENED ELECTROLYTIC TINPLATE IS SUBJECTED FOLLOWING FLOW BRIGHTENING. 